1. Field of the Invention
The present invention relates generally to aircraft having flexible airfoils, and more particularly, to a helicopter utilizing a plurality of segmented, flexible airfoils in place of conventional rotor blades, and to a pitch control mechanism particularly adapted thereto.
2. Description of the Prior Art Helicopters are widely used in both civilian and military applications wherein vertical take-offs and landings and the capability of hovering are desired. Nonetheless, the design of conventional helicopter rotors often results in undesirable features. For example, conventional rotor assemblies must be rotated relatively rapidly in order to generate sufficient lift to maintain the aircraft aloft. Tip speeds of rotor blades typically reach 85 percent of the speed of sound in air. As a result, helicopters tend to be rather noisy and unsuited for certain military applications wherein such noise puts others on notice of the presence of the helicopter. In addition, should the engine fail while such conventional helicopters are aloft, then a pilot must immediately make critical adjustments to the pitch of the rotor blades in order to attempt a landing via autogyration (autorotation) of the rotor to create lift for slowing the descent. During such an engine-out condition, a pilot may panic and fail to make such adjustments. Moreover, landings made via autogyration may be safely attempted only when the aircraft is approximately 50 feet to 200 feet from the ground; at altitudes of less than 50 feet, even an experienced pilot lacks the time needed to make critical pitch angle adjustments, while at altitudes above 200 feet, the gravitational acceleration of the aircraft far exceeds the lift created by autogyration.
Attempts to utilize a flexible airfoil in place of a rotor blade for a helicopter are known in the art. For example, U.S. Pat. No. 3,597,108 discloses a semi-rigid airfoil for use with a helicopter or the like and including a rigid leading edge spar and a cable defining a trailing edge while a flexible material extends between the leading and trailing edges to form the airfoil surface. Similarly, U.S. Pat. No. 3,188,020 discloses a helicopter having flexible rotor blades, each of which includes a pair of flexible cables forming leading and trailing edges of the blade and including a flexible membrane stretched therebetween. Analytical studies of such a flexible blade are reported in "An Investigation Of Extremely Flexible Lifting Rotors", by Winston, N.A.S.A. Technical Note D-4465, April, 1968, and "A Hovering Investigation Of An Extremely Flexible Lifting Rotor", by Winston, N.A.S.A. Technical Note D-4820, October, 1968.
Though such prior art flexible rotor blades serve to reduce the weight of the aircraft, these prior art flexible rotors must nonetheless be rotated at relatively high rates in order to develop sufficient lifting forces to allow the aircraft to take off and hover. Moreover, it is believed that the prior art has failed to teach a simple and reliable mechanism for varying the collective and/or cyclic pitch of such flexible rotor blades.
Non-rotational flexible airfoils are also known in the art for use with slow flight, non-hovering aircraft, such as kites, hang gliders, and ultra-light powered aircraft. One such slow flight wing has a generally triangular shape and completely flexible trailing edges; such prior slow flight wings are known as delta wings, parasol wings, and rogallo wings. Prior art references disclosing such airfoils include U.S. Pat. Nos. 2,546,078; 3,141,640; 3,194,514; 3,250,500; 3,361,388; 3,830,449; and Des. 224,248. Additional details concerning such airfoils are described in "The Complete Outfitting And Source Book for Hang Gliding", Library of Congress No. TL 766 M46. The aforementioned slow flight airfoils are generally symmetrical and include one or more flexible billowing segments for creating lift upon translational movement. Such symmetrically shaped airfoils are not adapted for use with rotary winged vehicles as the speed of each portion of the rotary wing varies depending upon its distance from the rotor axle.
Accordingly, it is an object of the present invention to provide a flexible helicopter rotor adapted to be rotated at relatively low speeds and providing a significantly quieter operation than helicopter rotors of the type already known to the art.
It is another object of the present invention to provide a flexible helicopter rotor adapted for an ultra-light rotary wing aircraft.
It is still another object of the present invention to provide a flexible helicopter rotor which may be safely autogyrated from any altitude.
It is a further object of the present invention to provide a flexible helicopter rotor which slows the descent of an aircraft by parachuting during non-autogyrating operation.
It is a still further object of the present invention to provide such a flexible helicopter rotor including a relaively simple and lightweight pitch change control mechanism for varying the collective and cyclic pitch of the flexible airfoils within such a rotor.
These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds.